This study presents a microelectrophoresis platform to measure the electrophoretic mobility and zeta potential of coacervates and in vitro condensate models. Unlike traditional methods such as laser Doppler electrophoresis, which assume size-independent electrophoretic mobility, this method accounts for the fluid nature of coacervates, revealing size-dependent mobility. The zeta potential is calculated using the theory of Ohshima et al. for polarizable liquid droplets. The platform enables the direct measurement of electrophoretic mobility at the single droplet level, overcoming limitations of previous techniques that suffer from artifacts due to sedimentation and droplet coalescence. The study shows that coacervates have a surface charge, with polylysine chains enriched at the surface of polylysine/polyaspartic acid complex coacervates, causing negatively charged α-synuclein to adsorb and accumulate at the interface. Addition of ATP inverts the surface charge, displacing α-synuclein and potentially suppressing its interface-catalyzed aggregation. These findings demonstrate how condensate surface charge can be measured and altered, making microelectrophoresis a promising technique for characterizing biomolecular condensates and coacervate protocells. The study also shows that various condensate systems, including those formed by disordered elastin-like peptides, ribonucleoprotein, nucleolus granular components, and highly charged protamines, exhibit nonzero surface charges. These surface charges influence interactions with cellular components, such as proteins, organelles, and other condensates. The surface charge of coacervates can be modulated by client molecules, such as ATP, which can suppress or invert the surface charge, affecting protein adsorption and aggregation. The microelectrophoresis platform provides a quantitative and systematic way to investigate the effects of different proteins and metabolites on condensate surface properties. This method allows for more accurate determination of zeta potentials of a wide range of condensates compared to traditional techniques, offering new insights into condensate properties and their interactions with other cellular components. The findings highlight the importance of surface charge in governing molecular exchange, accumulation of species at the interface, and the stability of condensates against coalescence.This study presents a microelectrophoresis platform to measure the electrophoretic mobility and zeta potential of coacervates and in vitro condensate models. Unlike traditional methods such as laser Doppler electrophoresis, which assume size-independent electrophoretic mobility, this method accounts for the fluid nature of coacervates, revealing size-dependent mobility. The zeta potential is calculated using the theory of Ohshima et al. for polarizable liquid droplets. The platform enables the direct measurement of electrophoretic mobility at the single droplet level, overcoming limitations of previous techniques that suffer from artifacts due to sedimentation and droplet coalescence. The study shows that coacervates have a surface charge, with polylysine chains enriched at the surface of polylysine/polyaspartic acid complex coacervates, causing negatively charged α-synuclein to adsorb and accumulate at the interface. Addition of ATP inverts the surface charge, displacing α-synuclein and potentially suppressing its interface-catalyzed aggregation. These findings demonstrate how condensate surface charge can be measured and altered, making microelectrophoresis a promising technique for characterizing biomolecular condensates and coacervate protocells. The study also shows that various condensate systems, including those formed by disordered elastin-like peptides, ribonucleoprotein, nucleolus granular components, and highly charged protamines, exhibit nonzero surface charges. These surface charges influence interactions with cellular components, such as proteins, organelles, and other condensates. The surface charge of coacervates can be modulated by client molecules, such as ATP, which can suppress or invert the surface charge, affecting protein adsorption and aggregation. The microelectrophoresis platform provides a quantitative and systematic way to investigate the effects of different proteins and metabolites on condensate surface properties. This method allows for more accurate determination of zeta potentials of a wide range of condensates compared to traditional techniques, offering new insights into condensate properties and their interactions with other cellular components. The findings highlight the importance of surface charge in governing molecular exchange, accumulation of species at the interface, and the stability of condensates against coalescence.